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PWM detector?

Hello all,

I am designing an automotive product (operates 9 to 16VDC) that will drive a regulated high current 10 amp load with a NPN low side drive transistor. It will maintain average current to the load via PWM (400Hz 1% to 99% duty cycle) and sense resistor.

My question, I want a way to shut the load down in the event the PWM controller flakes out and gets stuck high. This would be a bad thing since the load would remain on 100% and possibly be damaged. So, what I am asking is...

Is there a cheap and easy way to detect the presence of a PWM signal and allow operation, but, in the event the PWM gets stuck on for X amount of periods, turn the output stage off?

I tried coupling the output FET to the PWM drive circuit through a capacitor. However, no matter what value cap I choose, I compromise either high duty or low duty cycle drive. Is there a solid state solution that will not affect the waveform?

Thanks

Gerb

Reply to
Gerbermultit00l
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Retriggerable oneshot.

Reply to
MooseFET

t

Ok, you have my attention.

Would this work as an edge trigger? I guess what i am getting at, would it work reguardless of the duty cycle of PWM?

I'll look into it.

Thanks.

Gerb

Reply to
Gerbermultit00l

An alternative solution might be a Polyfuse or something similar... which could (depending on how sensitive the load is) also protect against output stage failure.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

** OK - so the switch is a BJT.

** Now the switch is a MOSFET.

????????????????

..... Phil

Reply to
Phil Allison

AC couple, but with a DC restoration diode.

in--------c------+------------+--------nfet gate | | | | | | R K diode | A | | | | | | gnd gnd

John

Reply to
John Larkin

Put a current sense in line with either the C/E and have it operate a comparator or some current sense transistor config to operate a 555 timer which will trip the Threshold input. The discharge output can then be used to disable some circuit..

Tailor an RC L circuit from the current sense for the Threshold input so that it reaches 2/3 Vcc voltage in the time it would take for 2 time periods of a PWM..

How this works, if the pulses stop and it locks on or the output transistor shorts, constant current will appear (Ice), and force the Threshold on the timer to trip thus putting the 555 into a latched state which will also set the discharge pin to low. You will need to reset the timer with a power up or button to restart it.

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Reply to
Jamie

Sorry everyone. My bad. N-channel MOSFET. Not an NPN MOSFET :)

Gerb

Reply to
Gerbermultit00l

"The Journey is the reward"

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eff.com- Hide quoted text -

I have not checked in a few years, but, I was unable to find a Polyfuse that could handle this high of a current (can sustain peaks of 25A for brief duration).

I did not mention this earlier, but, the temperature extremes I will be working with might rule a Polyfuse out. The circuit must operate in an environment -40C to 125C.

Gerb

Reply to
Gerbermultit00l

Is the output device a low side NPN, or a MOSFET ? (You mention both.)

Anyway, this gets pretty close to what you're asking for:

C1 R1 | |---------- || ___ | >---||--o-----o--|___|---| |

Reply to
James Arthur

The original (AFAIK) Signetics data_sheet_cum_app_note on the 555 featured a missing pulse detector circuit. Just googling "missing pulse detector" came up with a bundle of hits - inluding 555 circuits - that might steer the O/P.

This assumes that the extremes of his/her PWM wavefom lend themselves to this type of detector.

Reply to
rebel

You have had various suggestions, but do you actually want to do this? If it is a high average power that is dangerous, perhaps you should simply detect this directly. For example is a continuous 99% duty really that much safer than continuous 100%?

In which case a simple RC filter and comparator would work, assuming a basically resistive load:

o--------[R]------|\\ | | \\ ----- | ---- Alarm ----- | / | -|/ GND | REF

--

John Devereux
Reply to
John Devereux

t

First off let me thank everyone for their poropsals. A few good ideas came to light that will be useful in the future.

However, you are absolutely right John. It is the power to the load over time that is damaging. Not neccessarily the exact duty cycle.

My original thought was to shut things down should the micro flake out and peg the output high 100% on for prolonged durations. But, if the micro did the same thing, say at even 95%, the same net result would occur.

Thank you for letting me look at the problem from a different angle. Your proposed solution makes things perfectly clear.

Gerb

Reply to
Gerbermultit00l

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